The present invention relates to a monochromatic radiation source for use in optical processing of materials, e.g. lithography in the manufacture of semiconductors, and more particularly to a low-pressure mercury resonance radiation source utilizing the atomic mercury resonance line.
As a monochromatic radiation source, an excimer laser has recently been attracting attention. In molecular physics an excimer means a dimer molecule produced by polymerization of two atoms or molecules in which are bound together an atom or molecule in the excited state and an atom or molecule in the ground state. In particular, if a mixture of a rare gas and a halogen gas is excited by an electron beam or an electric discharge, an excimer is produced, and narrow band radiation is emitted when the excimer, which is in the excited state, returns to the ground state which subsequently dissociates. For example, an excimer laser (hereinafter referred to as "the Kr-F laser") using a combination of krypton (Kr) and fluorine (F) emits narrow band radiation having a central wavelength of 248 nm.
An ideal radiation source for use in lithography is characterized by an ultra narrow energy half-width of the emitted radiation. This characteristic reduces adverse chromatic aberration affects in an optical lens system receiving and transmitting the radiation from the radiation source. In the case of the wavelength of 248 nm, the energy half width expressed in nanometers must be reduced to approximately 0.003 nm. However, this has not been realized yet in the aforesaid Kr-F laser.
Further, since the duration of the excited state of an excimer in the excimer laser is as short as a few nano seconds, a laser beam is emitted in the form of a pulsed radiation by repetition of electric discharge using capacitors at a constant frequency (e.g. 200 Hz). Therefore, in order to properly expose with the proper photon dose a resist coated on a material to be processed, it is necessary to control both the individual laser beam pulses and the pulse-to-pulse repeatibility. This makes the exposure apparatus more intricate. Moreover, the pulses exciting the laser form a propagating electromagnetic wave, which is liable to adversely affect both the microelectronic and optical devices associated with the output laser pulse control apparatus.
Alternatively, monochromatic radiation may be obtained by filtering a radiation source having a plurality of spectra, which is generated for example by a mercury-xenon lamp, a xenon lamp, a low-pressure mercury lamp, or a like lamp. However, this method suffers from the problem of a large amount of energy being lost in the filtration and hence low efficiency.
Two of the present inventors previously proposed a monochromatic radiation source using the mercury resonance line (J. Phys. D, 1970, Vol. 3, pp. 607-610). This monochromatic radiation source employs both a mercury-argon discharge lamp and a mercury resonance cell combined in a single unit. This apparatus selectively emits ultraviolet radiation having a wavelengh in the vicinity of 253.7 nm by sequentially generating broad band radiation in the discharge lamp and introducing it into the resonance cell where it is absorbed selectively and reemitted in a narrow band.
However, this prior art ultraviolet radiation source has the disadvantage that the pressure of mercury vapor contained in the discharge lamp cannot be controlled, so that an undesired arc discharge frequently occurs, which results in wear of the electrodes and unstable intensity of the emitted radiation.